The present invention relates to a voltage source inverter and, more particularly, to a voltage source inverter having an improved control circuit.
The output voltage waveform of the voltage source inverter is preferably a sine wave except in special cases. Various conventional means for improving the output voltage waveform have been developed and have actually been put to use. In a voltage source inverter for obtaining an AC output voltage from a DC voltage of a constant value, a typical known method for controlling the output voltage waveform is a pulse width modulation (PMW) method, according to which a sine wave output voltage that is pulse-width-modulated may be obtained. FIGS. 1A and 1C are views of waveforms for explaining the principle for obtaining the modulated sine wave by the conventional PMW method. Referring to FIG. 1A, a reference voltage signal SR1 of a sinusoidal waveform is compared with a carrier signal CW of a triangular waveform. As a result, an ON signal to be applied to switching means included in a main circuit of the inverter for applying a positive voltage between load terminals of the inverter is obtained in correspondence with a positive half-wave of the reference signal SR1 as shown in FIG. 1B. Also, an ON signal to be applied to the switching means for applying a negative voltage between the load terminals of the inverter is obtained in correspondence with the negative half-wave of the reference signal SR1 as shown in FIG. 1C. A pulse-width-modulated output voltage as shown hatched in FIG. 1D is obtained by controlling the switch means of the main circuit of the inverter by the ON signals shown in FIGS. 1B and 1C. This modulated output voltage is equivalent to a sine wave voltage SR1. In other words, the sine wave voltage SR2 is obtained in correspondence with the sinusoidal reference voltage signal SR1 as the output voltage of the inverter.
With conventional means for obtaining the modulated sine wave, it is general practice to compare the reference voltage signal SR1 of the sinusoidal waveform with the carrier signal CW of the triangular waveform as has already been described. With such a method, since it is necessary to obtain the reference voltage signal SR1 in a sinusoidal waveform by multiplying the unit sine wave with the magnitude of the voltage instruction value, a multiplying operator is required. Further, when the frequency of the carrier signal CW of the triangular waveform is kept constant, the ratio of the carrier wave frequency to output voltage frequency decreases as the frequency of the output voltage increases. Consequently, a beat frequency is superposed on the output voltage frequency. In order to prevent such problems, the output voltage frequency and the carrier signal frequency must be synchronized. Further, when the output voltage frequency is controlled within a wide range, the output voltage frequency and the carrier wave frequency need to be controlled in relation to each other. Due to this, the method for modulating the sine wave SR1 by comparing the sinusoidal reference voltage signal with the triangular carrier wave has been defective in that the construction of the voltage source inverter becomes extremely complex. Further, it has been difficult to obtain a satisfactory output waveform, that is, an output waveform of good precision, due to the limits imposed by the construction of the control circuit.